In a certain mass spectrograph, an ion beam passes through a velocity filter consisting of mutually perpendicular fields: electric field E→ and magnetic field B→. The beam then enters a region of another magnetic field, B′→, which is perpendicular to the ion beam. The radius of curvature of the resulting ion beam will be proportional to what? B/EB′ BB′/E E/BB′ EB′/B EB/B′